Prompt suggestions on the new-chat screen

A blank composer is the least helpful thing a chat client can show someone
who has just installed one. Three cards now sit under the empty state, and
an administrator manages them at /admin/suggestions.

Clicking a card fills the composer and stops there. It deliberately does not
send: every default ends mid-sentence, because a card is a starting point
rather than a question somebody already asked, and the caret lands where the
person has to start typing.

Seeding is guarded by a settings flag, not by "is the table empty" --
otherwise an administrator who decided against them would get all three back
on every restart. Capped at twelve, six shown: past a dozen this is a menu,
and a menu on the empty screen is a worse blank page than a blank page.

The cards are gated on there being no chat at all, not on the thread being
empty. An empty chat someone opened on purpose already has a model and a
prompt chosen.

Also fixes a pre-existing bug the position test caught. Both this and
_refresh_models wrote `coalesce(max(position), -1) or -1`, and position 0 is
falsy -- so the second row landed back on 0 on top of the first. The
coalesce was already doing that job; the `or` was undoing it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/lembas/services/suggestions.py

@@ -0,0 +1,118 @@
+"""The cards offered on the new-chat screen.
+
+A blank composer is the least helpful thing a chat client can show someone who
+has just installed one. These are three starting points an administrator can
+replace with their own.
+"""
+
+from __future__ import annotations
+
+import logging
+
+from sqlalchemy import func, select
+from sqlalchemy.orm import Session as DBSession
+
+from lembas.db.models import Suggestion
+from lembas.services import settings_store
+
+log = logging.getLogger(__name__)
+
+# A short list stays a short list. Past a dozen it is a menu, and a menu on the
+# empty screen is a worse blank page than a blank page.
+MAX_SUGGESTIONS = 12
+MAX_SHOWN = 6
+
+MAX_NAME = 120
+MAX_DESCRIPTION = 300
+MAX_PROMPT = 4000
+
+# Each ends mid-sentence, so the caret lands exactly where the person has to
+# start typing. No Middle-earth flavour: this is functional UI.
+DEFAULTS: tuple[tuple[str, str, str], ...] = (
+    (
+        "Explain this",
+        "Paste something confusing and get it back in plain language.",
+        "Explain the following in plain language. Start with one sentence "
+        "summarising it, then the details that actually matter, then anything I "
+        "should watch out for. If I have not pasted anything yet, ask me for it "
+        "rather than guessing.\n\n",
+    ),
+    (
+        "Draft a reply",
+        "Turn a message you have received into an answer you can send.",
+        "Help me reply to the message below. If the tone I want and the outcome "
+        "I am after are not obvious from it, ask me before writing. Then give me "
+        "a draft I could send as it stands.\n\n",
+    ),
+    (
+        "Find the flaw",
+        "Have a plan argued with before you commit to it.",
+        "I am going to describe a plan. Argue against it: what is most likely to "
+        "go wrong, what am I assuming without evidence, and what would change "
+        "your mind. Do not soften it, and do not agree just because I sound "
+        "confident.\n\nMy plan: ",
+    ),
+)
+
+# Guards the seed. Not "is the table empty", because an administrator who
+# deletes all three would get them back on every restart.
+SEEDED_KEY = "suggestions_seeded"
+
+
+def visible(db: DBSession) -> list[Suggestion]:
+    """What the new-chat screen shows, in order."""
+    return list(
+        db.scalars(
+            select(Suggestion)
+            .where(Suggestion.enabled.is_(True))
+            .order_by(Suggestion.position, Suggestion.name)
+            .limit(MAX_SHOWN)
+        )
+    )
+
+
+def all_of_them(db: DBSession) -> list[Suggestion]:
+    """Every suggestion, enabled or not, for the admin page."""
+    return list(
+        db.scalars(select(Suggestion).order_by(Suggestion.position, Suggestion.name))
+    )
+
+
+def next_position(db: DBSession) -> int:
+    """New rows land at the end rather than at 0, where they would shuffle.
+
+    No `or -1` after the coalesce: position 0 is falsy, so that idiom sends the
+    second row back to 0 on top of the first.
+    """
+    highest = db.scalar(select(func.coalesce(func.max(Suggestion.position), -1)))
+    return int(highest if highest is not None else -1) + 1
+
+
+def create(db: DBSession, *, name: str, description: str = "", prompt: str = "") -> Suggestion:
+    suggestion = Suggestion(
+        name=name.strip()[:MAX_NAME],
+        description=description.strip()[:MAX_DESCRIPTION],
+        prompt=prompt[:MAX_PROMPT],
+        position=next_position(db),
+    )
+    db.add(suggestion)
+    db.commit()
+    return suggestion
+
+
+def seed_defaults(db: DBSession) -> int:
+    """Write the built-in suggestions, once ever.
+
+    Runs from the startup housekeeping block. The flag is what makes it once:
+    checking whether the table is empty would restore all three every restart
+    for anyone who decided they did not want them.
+    """
+    if settings_store.get(db, SEEDED_KEY):
+        return 0
+
+    for name, description, prompt in DEFAULTS:
+        create(db, name=name, description=description, prompt=prompt)
+
+    settings_store.update(db, {SEEDED_KEY: True})
+    log.info("seeded %d default suggestion(s)", len(DEFAULTS))
+    return len(DEFAULTS)